1. Field of the Invention
This invention relates to a method for forming a phosphate conversion coating on metal surfaces that include at least some aluminiferous metal surfaces, which for the purposes of the present invention are to be understood as those containing at least 0.5% of aluminum in their surfaces, are generally exemplified by aluminum sheet, steel sheet plated with Zn-Al alloy, aluminum-plated steel sheet, and the like, and are found, for example, on automotive outer body sheet, on colored sheet-metal roofing material, and so forth.
1. Statement of Related Art
Phosphate conversion coating of metals forms on the metal surface a coating, with a chemical composition typified by Zn.sub.3 (PO.sub.4).sub.2 .multidot.n H.sub.2 O, Zn.sub.2 Fe(PO.sub.4).sub.2 .multidot.n H.sub.2 O, and/or Zn.sub.2 Ca(PO.sub.4).sub.2 .multidot.n H.sub.2 O for the example of zinc phosphate treatments. This is brought about by contacting the metal surface, usually is by spraying or dipping, with a phosphate liquid conversion coating composition in order to dissolve the surface of the metal. Phosphate conversion coatings improve the metal's resistance to rusting and its paint adherence.
Zinc phosphate liquid conversion coating compositions contain phosphoric acid, Zn.sup.2+ ions, and water as their essential components and may contain, for example, Ni.sup.2+, Mn.sup.2+, Ca.sup.2+, Fe.sup.2+, H.sup.+, Na.sup.+, K.sup.+, NH.sub.4.sup.+, NO.sub.3.sup.-, NO.sub.2.sup.-, fluoride (which may be simple fluoride and/or complex fluorides such as fluozirocnate, fluosilicate, fluoborate, and the like), SO.sub.4.sup.2-, Cl.sup.-, or the like, as optional components. The listed components represent those in general use for phosphate liquid conversion coating compositions.
Phosphate sludge, i.e., FePO.sub.4 .multidot.n H.sub.2 O, Zn.sub.3 (PO.sub.4).sub.2 .multidot.nH.sub.2 O, etc., is produced during the phosphate conversion coating of metal surfaces. Sludge-eliminating measures such as filtration and sedimentation are therefore ordinarily used since a low sludge content in the liquid conversion coating composition is desirable for forming phosphate conversion coatings.
Zn-Al alloy-plated steel sheet exhibits a better corrosion resistance and heat resistance than ordinary Zn-plated steel sheet, and as a result its applications as a corrosion-resistant material have in recent years been undergoing active development. In particular, steel sheet plated with Zn containing approximately 5% of aluminum is widely used where painting is to be executed on the steel sheet. However, when this type of steel sheet is subjected to phosphate conversion coating, aluminum ions accumulate in the treatment bath. Removal of aluminum ions from the reaction system is desired because aluminum ions are unnecessary and in fact detrimental to phosphate conversion coating.
In order to prevent Al ion accumulation in the liquid conversion coating composition, it is desirable in the conversion coating of aluminiferous metal surfaces to remove Al ions from the reaction system as a solid. For this purpose, when conversion coating at least partially aluminiferous metal surfaces, a sludge such as NaK.sub.2 AlF.sub.6, Na.sub.3 AlF.sub.6, etc., may also be deliberately produced, in addition to the phosphate sludge which arises spontaneously during conversion coating of any ferrous and/or zinciferous surface. For the purposes of the present specification, the term "sludge" designates a mixture of (a) solids, such as the phosphates and aluminum compounds produced by the reactions occurring during phosphate conversion coating of the metal or produced by the addition of NaHF.sub.2, KHF.sub.2, etc., plus (b) small amounts of metal salts not due to the aforesaid reactions, plus (c) atmospheric dust and entrained contamination adhering on the metal. This sludge is therefore typically a mixture of components such as zinc phosphate, iron phosphate, aluminum compounds such as cryolite (Na.sub.3 AlF.sub.6) or elpasolite (NaK.sub.2 AlF.sub.6), entrained contaminants, dust, and the like.
Japanese Patent Application Laid Open Kokai or Unexamined! Number Sho 57-70281 70,281/1982! discloses a phosphate conversion coating method that maintains the Al ion concentration in a zinc phosphate liquid conversion coating composition at 50 parts per million (hereinafter usually abbreviated "ppm") or less. According to this disclosure, the Al elution into a zinc phosphate liquid conversion coating composition, expressed on the basis of the unit surface area of the steel sheet, is 0.2.times.10.sup.-3 to 1.0.times.10.sup.-3 moles per square meter, hereinafter usually abbreviated as "mol/m.sup.2 ". According to this method, at this quantity of elution the Al ion can be maintained at 50 ppm or less by removal of the Al as a precipitate through the continuous supply of NaHF.sub.2 equimolar with the eluting Al (=0.2.times.10.sup.-3 to 1.0.times.10.sup.-3 mol/m.sup.2) and KHF.sub.2 at twice the moles of the eluting Al (=0.4.times.10.sup.-3 to 2.0.times.10.sup.-3 mol/m.sup.2). The prescribed Al ion concentration is obtained in this method by the precipitative removal of Al from the reaction system as elpasolite (NaK.sub.2 AlF.sub.6).
Japanese Patent Application Laid Open Kokai or Unexamined! Number Sho 60-204889 1204,889/1985! discloses a phosphate conversion coating method that is directed to zinc phosphate conversion coating liquid compositions whose principal ingredients are 0.85 to 1.7 grams per liter (hereinafter usually abbreviated as "g/L") of Zn ions, 1.0 to 2.5 g/L of Ni ions, 5.5 to 10 g/L of PO.sub.4 ions, 0.8 to 1.5 g/L of N0.sub.3 ions, 1.3 to 5.5 g/L of F ions, and small quantities of additives. In this method, NaHF.sub.2 and KHF.sub.2 --or NaHF.sub.2 and KF--are intermittently added to such liquid compositions in a quantity no more than equimolar with the quantity of Al in the liquid conversion coating composition. This results in the sedimentative removal of the Al in the zinc phosphate liquid conversion coating composition as K.sub.2 NaAlF.sub.6 and provides control of the Al ions concentration in the liquid conversion coating composition to 500 ppm or less.
Removal of the aluminum ions eluted from zinc-aluminum alloy plating during phosphate conversion coating as cryolite (Na.sub.3 AlF6) is also known and is described in Dai-39-kai Fushoku Boshoku Toronkai Gaiyo Title in English: Proceedings of the 39th Japan Corrosion Conference! in Fushoku Boshoku Kyokaishi Title in English: Corrosion Engineering!.
Japanese Patent Application Laid Open Kokai or Unexamined! Number Sho 56-105485 105,485/1981!teaches a method that facilitates sludge filtration. In this method, a polymeric coagulant is added to the conversion coating liquid composition in the phosphate conversion coating of cold-rolled steel sheet. The addition of this polymeric coagulant improves the sedimentability of the phosphate compound-containing sludge. However, this method was not investigated with regard to the sedimentation of aluminum compound-containing sludge.
The sedimentation of zinc phosphate-based sludge is facilitated by the addition of polymeric coagulant in the method taught in Japanese Patent Application Laid Open Kokai or Unexamined! Number Sho 56-105485 for the formation of zinc phosphate conversion coatings on steel sheet or zinc-plated steel sheet. However, there are believed to have been no previous investigations into a method that could induce the sedimentation of sludge from a liquid conversion coating composition in which aluminum compounds are present.